Sensors used with automatic door openers to sense the motion of a person or an animal in a field of view are well known in the art. These sensors typically use microwave transceivers to detect motion. Microwave detectors sense the doppler shift of the reflected microwave signal from a moving target to detect motion. Other motion sensors include passive infrared detectors.
Of the two types of sensing technology, passive infrared detectors and microwave detectors, passive infrared detectors are smaller and less complicated. Thus, they are less expensive than microwave detectors. However, passive infrared detectors are subject to noise near ambient body temperature. In addition, these sensors are sensitive to sunlight.
In contrast, microwave detectors are more immune to temperature variations and other environmental conditions such as sunlight and drift. The problem with motion detectors which actuate the opening of doors is that the sensors do not detect the presence of an individual--only the motion of an individual. Thus, for example, if a person stops while moving towards the threshold of an automatic door, the microwave detector would stop detecting the motion and hence would conclude that the person has crossed the threshold of the automatic door. The door would then be closed, posing a safety problem. The individual may be knocked down by the closing of the door if the individual is directly in the path of the threshold of the doorway or may be frightened by the closing of the door.
To overcome the problem of a person stopping near the threshold of the door, active infrared detectors have been used in combination with microwave detectors. The active infrared detector is positioned near the threshold of the doorway. The active infrared detector, emits infrared radiation which is received by a sensor placed at the opposite location of the doorway threshold. The presence of a person in that position blocking the infrared radiation would cause the active infrared detector to sense the presence of the individual.
Long delays are added to motion detectors before the doors are closed because the motion detector cannot detect when a person is near the threshold of a doorway. This condition occurs when an elderly or a slowly moving person requires more time to cross the doorway threshold. Since the active infrared detector, even if used with a microwave detector, is always positioned across the threshold of a doorway, the active infrared detector has a narrow field of view. The result is that delays are added to keep the doors open longer irrespective of the pace of the person or animal to prevent mishaps. The maintainence of the door in an open position unnecessarily creates energy loss from within the building to the external environment.